SWI-Prolog -- explain/2

Documentation
- Reference manual
  - Overview
    - Online Help
      - help/0
      - help/1
      - apropos/1
      - explain/1
      - explain/2
- Packages

Availability::- use_module(library(explain)).(can be autoloaded)

Source

explain(+ToExplain, -Explanation)

Unify Explanation with an explanation for ToExplain. Backtracking yields further explanations.